A material of excellent thermal insulation properties can be made from a mixture of finely divided microporous silica aerogel, a reinforcing fibre such as alumina silicate and optionally a separate particulate opacifier. Such materials give excellent heat insulation when compacted under pressure and self bonded with or without the use of extra bonding agents to a density in the range 10 to 30 lb/ft.sup.3 and are useful also as a base layer of thermal and electrical insulating material such as in heating units for smooth top electric cooker hobs as illustrated, for example, in our United Kingdom Pat. No. 1433478.
When such a material is used at high temperatures it is found to sinter and consequently shrink. This is most undesirable since shrinkage leads to the formation of cracks and air gaps whose insulating properties are much inferior to the material itself. Materials such as described above have a limiting temperature at which they are useful of about 1000.degree. C. since above that temperature there is progressive sintering and shrinkage.
Until now it had been assumed that such temperatures were the highest temperature at which these materials could be useful. This view was based on the facts that, of the three components noted above, silica, even when it is used in a form which is as pure as possible, has the lowest temperature at which it starts to sinter and shrink significantly, and it is present in the mixture in by far the largest amount both by weight and by volume.